On our boat which had lots of white seats, the gentleman that used do all the detailing told us of a 50/50 mix of Clorox Cleanup & water. It works unbelievably and alo use a scrub brush to get into the tiny little grooves of vinyl material.

I work in the marine industry and you guys are killing you vinyl seats. Don't use anything with a chlorine bleach, it will attack the stitching and all your seams will come apart. Also no 409 or armorall. No sos pads, nothing abrasive and no solvents. I would suggest going using a good marine vinyl cleaner and a soft brush. For specific stains research online. Do not use any potroleum products. This information is from many years dealing with the vinyl manufacturers.